Classification of Watershed
A. A large numbers of terms are very frequently and loosely used to classify watershed in different sizes [based on size]
a. Micro watersheds
b. small watersheds
c. Large watersheds, etc
“Small watersheds are those where the overland flow is the main contributor to peak runoff / flow and channel characteristic do not affect the overland flow”.
“Large watersheds are those give peak flows are greatly influenced by channel characteristics and basin storage”.
B .Watersheds is also classified into different categories based on area that the watersheds contain:
Type of Watershed
0 to 10 ha
10 to 40 ha
40 to 200 ha
200 to 400 ha
400 to 1000 ha
above 1000 ha
C. Classification based on shape:
a. Square b. Triangular c. Rectangular
d. Oval e. Fern leaf shaped f. Palm shaped
g. Polygon shaped h. Circular i. Secator shaped
Function of watershed:
The main function of watershed is to receive the incoming precipitation and then dispose it off. This is the essence of soil and water conservation.
Morphological characteristics of watershed:
Each individual watershed has several remarkable characteristics, which affect its functioning. Seven such characteristic have been identified.
Geology, rock and soil
Size of watershed determines the quantity of rainfall received retained and disposed off [runoff]. Larger the watershed, larger be the channel and storage of water in basin. Large watershed characteristics are topography, geology, soil, climate and use and vegetation.
Watershed may have several shapes like square triangular rectangular, oval, palm, fern leaf shape etc.
Shape of watershed determines the shape index [form factor Ff = WB/Lb]
That is the length: width ratio which in turn has a great effect on runoff disposal. Larger the watershed, higher is the time of concentration and more water will infiltrate, evaporate or get utilized by the vegetation. Reverse is the situation when watershed is shorter in length as compared to width.
Compactness coefficient Cc:
“Compactness coefficient of a watershed is the ratio of perimeter of watershed to circumference of circular area which equals the area of the watershed”. The C.C. is independent of size of watershed and dependent only on the slope.
Slope, length, degree and uniformity of slope affect both disposal of water and soil loss. Degree and length of slope also affect time of concentration [Tc] and infiltration of water.
Drainage: Topography regulates drainage. Drainage density [length of all drainage channels – unit area], length, width depth of main and subsidiary channel, main outlet and its size depend on photography. Drainage pattern affect time of concentration.
4. Geology rock and soil:
Geological formation and rock types affect extent of water erosion, erodibility of channels and hill faces, sediment production. Rocks like shale’s, phyllites erode easily where as igneous rocks do not erode.
Physical and chemical properties of soil, specially texture, and structure and soil depth influence disposition of water by way of infiltration, storage and runoff.
Climate parameters affect watershed functioning and its manipulation in two ways.
Rain provides incoming precipitation along with its various characteristic like intensity, frequency and amount of rainfall.
Parameters like rainfall, temperatcive, humidity, wind velocity, etc. regulates factors like soil and vegetation.
Depending upon the type of vegetation and its extent, this factor regulates the functioning of watershed ex. Infiltration, water retention, runoff production, erosion, sedimentation etc.
7. Land use:
Type of land use, its extent and management are the key factors which affect watershed behavior. Judicious land use by users [human beings] is of vital importance to watershed management and functioning.